1. Field of the Invention
The present invention relates to audio coding and decoding apparatuses and methods, and recording media storing the methods, and more particularly, to audio coding and decoding apparatuses and methods which support fine granularity scalability (FGS) using harmonic information of a high-band audio signal or wideband error audio signal when performing wideband audio coding and decoding, and a recording media storing the methods.
2. Description of Related Art
As the range of applications of audio communications and the transmission speed of networks have increased, the demand for high-quality audio communications has also increased. As such, while a conventional audio communication band is 0.3-3.4 kHz, a transmission of a wideband audio signal having a bandwidth of 0.3-7 kHz with high performance in a variety of aspects such as, for example, a natural property and clarity is needed.
In addition, a packet switching network via which data is transmitted in packet units may cause congestion of a channel and packet loss and audio degradation may occur. To solve this problem, a method of concealing a damaged packet has been used but this cannot be a fundamental solution.
Thus, a wideband audio coding and decoding method in which congestion of a channel is prevented by effectively compressing the wideband audio signal has been proposed.
Three examples of wideband audio coding and decoding methods include a first wideband audio coding and decoding method in which an audio signal having a bandwidth of 0.3-7 kHz is compressed at one time and restored, a second wideband audio coding and decoding method in which an audio signal having a bandwidth of 0.3-4 kHz and an audio signal having a bandwidth of 4-7 kHz are compressed hierarchically and restored, and a third wideband audio coding and decoding method in which an audio signal having a bandwidth of 0.3-3.4 kHz is compressed, restored and up-sampled to a wideband signal and a wideband error signal between an original wideband audio signal and the up-sampled wideband signal is obtained and compressed.
The second and third wideband audio coding and decoding methods use bandwidth scalability that enables optimum communication in a channel environment obtained by adjusting the amount of data of a layer to be transmitted according to the degree of congestion.
In the second and third wideband audio coding and decoding methods using the bandwidth scalability, a high-band audio signal having a frequency band of 4-7 kHz is coded using a modulated lapped transform (MLT). A high-band audio signal coding apparatus using a MLT is as shown in FIG. 1.
Referring to FIG. 1, if a high-band audio signal is inputted to the high-band audio signal coding apparatus, the high-band audio coding apparatus performs an MLT on the high-band audio signal inputted to an MLT unit 101 and extracts an MLT coefficient. The magnitude of the extracted MLT coefficient is outputted to a 2 dimensional discrete cosine transform (2D-DCT) module 102, and the sign of the extracted MLT coefficient is outputted to a sign quantizer 103.
The 2D-DCT module 102 extracts a 2D-DCT coefficient from the magnitude of an inputted MLT coefficient and outputs the extracted 2D-DCT coefficient to a DCT coefficient quantizer 104. The DCT coefficient quantizer 104 arranges 2D-DCT vector coefficients in an ascending series statistically, quantizes the arranged vectors and then outputs codebook indices of the arranged vectors. The sign quantizer 103 quantizes a sign of a large MLT coefficient and outputs the quantized sign. The outputted codebook indices and the quantized sign are provided to a high-band audio decoding apparatus (not shown).
However, in high-band audio signal coding using the MLT, it is difficult to restore a high-quality audio signal when an audio signal is transmitted at a low bit rate.
In order to solve this problem, a high-band audio coding apparatus using a harmonic coder shown in FIG. 2 has been proposed.
Referring to FIG. 2, a harmonic peak detector 201 detects a harmonic peak of the inputted high-band audio signal and outputs an amplitude and a phase of the high-band audio signal based on the detected harmonic peak.
An amplitude quantizer 202 quantizes the amplitude of the inputted high-band audio signal and outputs a high-band audio signal having the quantized amplitude. A phase quantizer 203 quantizes phase of the inputted high-band audio signal and outputs a high-band audio signal having the quantized phase. The quantized amplitude and the quantized phase are provided to a high-band audio decoding apparatus (not shown).
A high-quality signal can be reproduced at a low bit rate with low complexity through high-band audio signal coding using the harmonic coder shown in FIG. 2. However, there is a limited support of scalability for the inputted high-band audio signal.
In addition, when performing wideband error audio coding using the third method having the bandwidth scalability function, a wideband error audio signal having a bandwidth of 0.05-7 kHz is coded using a modified discrete cosine transform (MDCT). Awideband error audio signal coding apparatus using an MDCT shown in FIG. 3.
Referring to FIG. 3, if a wideband audio signal is inputted to the wideband error audio coding apparatus, the wideband error audio coding apparatus obtains a signal down-sampled to a low band using a down-sampling module 301 and codes the signal down-sampled to the low band using a low-band audio coder 302. The coded audio signal is restored to a wideband signal using an up-sampling module 303, and the restored wideband signal is subtracted from the inputted wideband audio signal by a subtracter 304 to generate a wideband error audio signal. The generated wideband error audio signal is inputted to an MDCT unit 305, and the MDCT unit 305 extracts an MDCT coefficient of the inputted wideband error audio signal. The extracted MDCT coefficient is divided into bands by a bandwidth dividing module 306, and the divided MDCT coefficient is normalized by a normalization module 307. The normalized MDCT coefficient is quantized by the quantizer 308, and the quantizer 308 outputs codebook indices. The outputted codebook indices are provided to a high-band audio decoding apparatus (not shown).
However, when an audio signal is transmitted at a low bit rate when using the wideband error audio signal coding method with the MDCT, it is difficult to restore a high-quality audio signal.